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Thermal performance of a section of the MacKenzie Highway
--
Ser
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TH1
1
N21d
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National Research Conseil national
no.
1173
1
Council Canada
de recherches Canada
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2
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BLl3G
THERMAL PERFORMANCE OF A SECTION OF THE MACKENZIE HIGHWAY
by L.E. Goodrich
Reprinted from
Permafrost: Fourth International Conference
Fairbanks, Alaska, July 17
-
22,1983
Proceedings, p. 353
-
358
DBR Paper No. 1173
Division of Building Research
On a Studi'e pendant c l n q ans l e comportentent thermique d'un remblai de r o u t e c o n s t r u i t s u r
l e
p e r g E l i s o 1 dans l a zone d i s c o n t i n u e d e l a vall'ee du Mackenzie. Ce remblai d e f a l b l e h a u t e u r (1,2 m) B t a i t f o r & de m a t k i a u de remblai propre, d'evers'e s u r une p a r t i e d'emprise p r h l a b l e m e n t dsbarrass'ee d e s e s a r b r e s . La r o u t e ~ ' a y a n t jamais (Et'e o u v e r t e3
l a grandec i r c u l a t i o n , l e s a c t i v i t ' e s de d h e l g e m e n t B t a i e n t r a u i t e s au minimum.
Les mesures p r i s e s comportalent l l e n r e g l s t r e m e n t h o r a i t e d e s temp'eratures du s o l
1
l ' a i d e d'un a p p a r e i l e n r e g l s t r e u r autonome. Les tassements o a t Bt'e c o n t r a l b e t des sondes de conductivit'e thermique i n s t a l l ' e e s pour o b t e n i r d e s donn-ks s u r l e s tendances s a i s o n n i 3 r e s dans l e remblai mSme e t dans l e s d i v e r s matsriaux composant l a couche a c t i v e .Les donn'ees obtenues indiquent que l a d6gradation du p e r @ i s o l e s t en c o u r s e t f o u r t i i s s e n t d e s renseignements s u r l e r a p p o r t e n t r e l e s temp'eratures de s u r f a c e d e l a r o u t e e t
l e s
f a c t e u r s c l i m a t i q u e s de l a rggion.Reprinted from:
Permafrost: Fourth International Conference, Proceedings
ISBN 0-309-03435-3 National Academy Press
Washington,
D.C.
1983THERMAL
PERFORMANCE OF A SECTION OFTHE
MACKENZIE HIGHWAYL. E. Goodrich
Division of Building Research, National Research Council of Canada, Ottawa, Ontario KIA OR6
The thermal performance of a shallow road embankment constructed on permafrost in the discontinuous zone of the Mackenzie Valley was monitored over a 5year period. The shallow (1.2 m) embankment consisted of clean borrow material end dumped on a right-of-way which had previously been cleared of trees. The road was never opened to general traffic; consequently snow removal was minimal. Measurements included hourly ground temperatures recorded with a data logger operating unattended. Settlements were monitored and thermal conductivity probes were installed to yield information on seasonal trends within the embankment and in the various natural materials comprising the active layer. The data indicate that degradation of the permafrost is under way and provide information on the relation between road surface temperatures and climatic factors for this region.
INTRODUCTION
In 1972 the Mackenzie Highway Environmental Working Group was established by the Department of Indian Affairs and Northern Development to monitor the environmental impact of the proposed Mackenzie Highway joining Fort Simpson to Inuvik and the Arctic coast. The Division of Building Research of the National Research Council of Canada was invited to carry out a limited study of the thermal aspects. At that time no case studies of shallow embankments constructed on permafrost had been carried out in the Fort Wrigley region. It was expected that although degradation would not pose serious problems in the zone south of Fort Wrigley, the data
collected could be used to anticipate problems in the more difficult soils to the north. Although, as originally planned, the road was to be a graveled, all-weather road, construction was halted and during the final two winters of this study there was only minimal snow removal.
OBJECTIVES
The specific objectives of this study were: 1. to observe the'changes in temperature regime in the ground underlying the road embankment as well as in the surrounding right-of-way in order to assess the thermal disturbance caused by the embankment;
2. to collect data on temperatures within the embankment for design purposes;
3. to assess the settlements of the embankment;
4.
to obtain basic soil information for the site, including profiles of ice content, soil density, and composition; and5. to measure thermal conductivities both in the natural ground and in the embankment and to assess seasonal and yeartcryear variations.
FIELD INSTRUMENTATION
Field work was begun in March 1976 at mile 419.5, near the northern end of the proposed new construction on the Mackenzie Highway (Figure 1). Earlier reconnaissance had indicated the presence of permafrost. This location also offered important logistic advantages. The Hire North camp at River Between-Two-Mountains could serve as base of operations, and in addition, a winter road facilitated access to Fort Simpson, the nearest point with land connections to the south. For these reasons this site was selected, even though more sensitive terrain could be found to the north.
To compare thermal c o n d i t i o n s under t h e f u t u r e embankment c e n t e r l i n e , t h e shoulder, t h e p r e v i o u s l y c l e a r e d right-of-way, and t h e o r i g i n a l u n d i s t u r b e d t e r r a i n , f o u r h o l e s were d r i l l e d approximately 8 m
deep ( t h e maximum a c h i e v a b l e w i t h equipment a t hand). These l o c a t i o n s correspond t o p r o f i l e s A, B, C , and D i n F i g u r e 2. Nearly continuous c o r e was recovered from h o l e A ( c e n t e r l i n e ) , and g r a b samples were c o l l e c t e d from h o l e s B and D. The s o i l t y p e , moisture c o n t e n t , and d e n s i t y were determined on s i t e . I n a d d i t i o n , r e p r e s e n t a t i v e c o r e samples were shipped t o t h e l a b o r a t o r y f o r f u r t h e r e v a l u a t i o n , i n c l u d i n g thermal c o n d u c t i v i t y measurements. The h o l e s were instrumented w i t h Yellow Springs #44033 t h e r m i s t o r s mounted on multiconductor c a b l e t o a depth of 7 m. The c a b l e s were cased i n PVC p l a s t i c t u b i n g f o r p r o t e c t i o n from mechanical damage, and t h e boreholes were b a c k f i l l e d w i t h d r y sand. A p r e f a b r i c a t e d instrument h u t made of f i b e r g l a s - Styrofoam sandwich p a n e l s was e r e c t e d a t t h e s i t e .
-
N 0B.M I-r
:!
1 0 1 0 N A T U R A L F O R E S T FIGURE 2 S i t e l a y o u t .t h e road and s t a k e m o u n t e d t h e r m i s t o r probes, s i m i l a r t o t h o s e used i n t h e shallow p i t s , were i n s t a l l e d i n t h e t r e n c h w a l l (Cables
H
and J i n F i g u r e 2). The a d d i t i o n of f o u r thermal c o n d u c t i v i t y probes completed t h e t h e r m a l i n s t r u m e n t a t i o n . F i n a l l y , s e t t l e m e n t p l a t e s were i n s t a l l e d on t h e c e n t e r l i n e and n e a r t h e s h o u l d e r t o monitor t o t a l s e t t l e m e n t of t h e embankment.The t e s t s i t e was operated, w i t h some
i n t e r r u p t i o n s , from September 1976 u n t i l March 1981. During t h i s time t h e data-logging equipment and t h e r m o e l e c t r i c power supply were l e f t t o f u n c t i o n unattended f o r p e r i o d s of 3-4 months. Approximately 80 channels, i n c l u d i n g 3 used t o v e r i f y t h e
equipment, were recorded every hour. Thermal c o n d u c t i v i t y measurements and l e v e l surveys were made whenever p o s s i b l e d u r i n g s i t e v i s i t s .
RESULTS
Figure 3 shows t h e s o i l p r o f i l e f o r t h e c e n t e r l i n e borehole ( p r o f i l e A). Conditions a t t h e t h r e e remaining boreholes were s i m i l a r , a s i n f e r r e d from s p l i t spoon sampling.. The p r o f i l e s c o n s i s t p r i m a r i l y of s i l t s and sands w i t h some l a y e r s of gravel. F i n e g r a i n e d m a t e r i a l s a r e p r e v a l e n t i n t h e upper p a r t of t h e p r o f i l e , w h i l e t h e g r a v e l l a y e r s a r e found only below 4 m. Correspondingly, t h e t o t a l volumetric moisture c o n t e n t v a r i e s from n e a r l y 80% i n t h e upper l e v e l s t o l e s s t h a n 40% a t depth. While some s m a l l i c e l e n s e s (<5 mm) a r e p r e s e n t i n t h e s i l t l a y e r s , most of t h e i c e i s d i s p e r s e d , and t h e s o i l i s w e l l bonded w i t h i c e throughout. MOISTURE VOLUME.
*
DENSIN, t/m3A Monitor Systems 9400 d a t a logger using a E
lower power Digi-Data t a p e d r i v e was i n s t a l l e d i n
September 1976. Power was provided by a 3M propane-
$
4 d r i v e n t h e r m o e l e c t r i c g e n e r a t o r which simultaneouslycharged a bank of Gel-Cell b a t t e r i e s s e r v i n g a s a
backup power supply. A propane f u r n a c e was I
i n s t a l l e d t o maintain ambient temperatures w i t h i n t h e h u t a t l e v e l s adequate f o r t h e data-logging
equipment. A t t h i s time a l s o , t h r e e small p i t s (E, h
F, and G i n F i g u r e 2) were dug t o a depth of
approximately 1 m. The p i t s were instrumented w i t h 7
MOSS
PEAT
SILT WITH ORGANIC DEBRIS
FINE SAND SOME SILT & C L A Y
MEDIUM TO COARSE
SOME GRAVEL LAYERS
SANDY SILT MEDIUM TO COARSE SAND COARSE SAND w l m GRAVEL t h e r m i s t o r s p o t t e d i n t o m e t a l t u b e s mounted h o r i z o n t a l l y on v e r t i c a l wooden s t a k e s . These i n s t a l l a t i o n s were i n t e n d e d t o provide more r e l i a b l e information on near-surface temperature c o n d i t i o n s t h a n t h a t a v a i l a b l e from t h e deep c a b l e s A-D. The p i t s were a l s o instrumented w i t h thermal
c o n d u c t i v i t y probes (Goodrich 1979) t o measure t h e s e a s o n a l v a r i a t i o n of c o n d u c t i v i t y f o r t h e d i f f e r e n t m a t e r i a l s comprising t h e a c t i v e l a y e r .
I n June 1977 t h e f i n a l grade was completed p a s t t h e instrumented s e c t i o n . A t r e n c h was dug a c r o s s
FIGURE 3 S o i l conditions: m i l e 419.5.
Daily mean temperatures f o r a l l channels were computed from hourly d a t a recorded on t h e o r i g i n a l tapes. Data p r o c e s s i n g i n c l u d e d a check of e a c h scan f o r instrument d r i f t , channel sequence, range and s i g n b i t , and c l o c k e r r o r s . I f more t h a n f i v e u n s a t i s f a c t o r y s c a n s occurred i n a d a i l y p e r i o d , t h e
e n t i r e d a i l y r e c o r d was r e j e c t e d . S i n c e t h e manufacturer claimed an i n t e r c h a n g e a b i l i t y e r r o r of
l e s s t h a n +O.l°C, t h e r m i s t o r l i n e a r i z a t i o n was done u s i n g t h e same formula f o r a l l channels.
Examination of F i g u r e 4 r e v e a l s , however, t h a t o v e r a l l e r r o r s may be c o n s i d e r a b l y g r e a t e r . The temperatures f o r channels 8-10, a l l of which a r e l o c a t e d a t l e v e l s w i t h i n t h e a c t i v e l a y e r ( a l s o channel 11 from 1979 onwards), should be near o r a t t h e f r e e z i n g p o i n t d u r i n g t h e autumn freezeback p e r i o d (zero-curtain e f f e c t ) . Although t h i s i s w e l l v e r i f i e d for. channels 8 and 10, t h e e r r o r f o r channel 9 i s r a t h e r more t h a n 1°C, and t h a t f o r channel 11 i s n e a r l y as g r e a t . It can a l s o be seen t h a t t h e e r r o r s i n c r e a s e w i t h time. E r r o r s of s i m i l a r magnitude were found i n o t h e r p r o f i l e s f o r s e v e r a l channels i n t h e a c t i v e l a y e r where a well- d e f i n e d zero-curtain e x i s t e d ; n e a r l y a t h i r d of t h e s e channels r e q u i r e d c o r r e c t i o n s of more t h a n 0.2OC. For channels below t h e a c t i v e l a y e r t h e r e i s no c e r t a i n r e f e r e n c e temperature. I n a few c a s e s , c o r r e c t i o n s could be i n f e r r e d from an examination of t h e annual mean p r o f i l e s a l o n g w i t h o b s e r v a t i o n of t h e temporal behavior. I n most i n s t a n c e s , however, t h e d a t a f o r t h e s e p o i n t s were r e j e c t e d . T y p i c a l c o r r e c t e d d a t a a r e shown f o r t h e c e n t e r l i n e p r o f i l e i n F i g u r e 5. T I M E , y e a r s FIGURE 4 E r r o r s i n raw d a t a of p r o f i l e A C/L. T I M E , y e a r s FIGURE 5 Corrected p r o f i l e A C/L. As r e g a r d s e q u i p e n t r e l i a b i l i t y , F i g u r e s 4 and
5 i n d i c a t e t h a t a n e a r l y complete record was obtained, w i t h two major exceptions. The l o n g gap from mid-November 1977 t o mid-September 1978 was t h e r e s u l t of t h r e e s e p a r a t e circumstances. The d a t a l o g g e r had t o be removed f o r r e p a i r s i n November 1977. A l l t h e equipment, e x c e p t t h e d e f e c t i v e d a t a l o g g e r , was l o s t when f i r e destroyed t h e s i t e i n March 1978. F u r t h e r d e l a y r e s u l t e d when p l a n s t o abandon c o n s t r u c t i o n of t h e road were announced. The second major gap, d u r i n g e a r l y 1979, was p u r e l y t h e r e s u l t of human e r r o r and not of equipment malfunction.
Thermal c o n d u c t i v i t y measurements were made whenever p o s s i b l e d u r i n g r o u t i n e s i t e v i s i t s .
F i f t e e n probes were e v e n t u a l l y emplaced i n a v a r i e t y of m a t e r i a l s , i n c l u d i n g t h e n a t u r a l l i c h e n m a t and t h e decomposed p e a t l a y e r , a s w e l l a s a t s e v e r a l d e p t h s i n t h e n a t u r a l s o i l and i n t h e embankment. Figure 6a shows r e s u l t s obtained both i n n a t u r a l p e a t and i n t h e p e a t l a y e r u n d e r l y i n g t h e
embankment. Data f o r a l l y e a r s have been p l o t t e d on t h e same axes. The d a t a can, i n both c a s e s , be reasonably w e l l r e p r e s e n t e d by c o n s t a n t f r o z e n o r thawed values. The thermal c o n d u c t i v i t y of t h e compressed p e a t beneath t h e embankment i s 2-3 times
t h a t of t h e n a t u r a l peat. I n a d d i t i o n , t h e r a t i o of f r o z e n t o thawed c o n d u c t i v i t y i s reduced from approximately 4 f o r t h e n a t u r a l p e a t t o
approximately 2 f o r t h e compressed peat. Figure 6b shows thermal c o n d u c t i v i t i e s measured n e a r t h e s u r f a c e and near t h e base of t h e embankment. The d a t a s u g g e s t t h a t f o r t h e l e v e l n e a r t h e s u r f a c e t h e thawed c o n d u c t i v i t y i s remarkably s i m i l a r from one y e a r t o t h e next. Frozen v a l u e s a r e d i s t i n c t l y g r e a t e r but a l s o show more s c a t t e r . It i s i n t e r e s t i n g t o n o t e t h a t f o r t h e lower l e v e l no d i s t i n c t s e a s o n a l p a t t e r n i s e v i d e n t . The g r e a t e s t y e a r t c r y e a r v a r i a t i o n i s , however, a g a i n found i n t h e w i n t e r values. J U L I A N D A Y S J U L I A N D A Y S
.
T O P OF F I L L o B A S E OF F I L L 8 0 0 0 - 0 o o 0 Q 8 8 . '..
0 l 8 l . FIGURE 6 Thermal c o n d u c t i v i t y v a r i a t i o n . .Some l e v e l survey r e s u l t s f o r t h e embankment c e n t e r l i n e a r e summarized i n F i g u r e 7. The curves suggest s i m i l a r t r e n d s a t t h e embankment s u r f a c e and base. (The i n c r e a s e i n s u r f a c e l e v e l i n mid-1979 r e f l e c t s road maintenance o p e r a t i o n s . ) T o t a l s e t t l e m e n t from c o n s t r u c t i o n i n mid-1977 t o t h e
f i n a l s u r v e y i n mid-1980 was approximately 0.3 m.
A t many s t a t i o n s i n t h e 200 f t (-60 m) s e c t i o n surveyed, t h e r e was c o n s i d e r a b l y more s e t t l e m e n t near t h e s h o u l d e r s t h a n on c e n t e r l i n e . By mid-1980 t h e road s u r f a c e had become a s e r i e s of bumps and hollows w i t h e l e v a t i o n d i f f e r e n c e s of approximately 0.15 m over 10 m i n t e r v a l s . - 0 . 3 1 1 I I I 7 6 17 7 8 7 9 80 8 1 TIME, y e a r s FIGURE 7 S e t t l e m e n t : C/L STN 1285
+
17. DISCUSSION Hand probing c a r r i e d o u t d u r i n g t h e i n i t i a l r e c o n n a i s s a n c e (autumn 1975) i n d i c a t e d t h a t t h e a c t i v e l a y e r i n a d j a c e n t u n d i s t u r b e d t e r r a i n a t t h e s i t e was t y p i c a l l y a b o u t 50 cm t h i c k . There was, however, evidence t h a t s i g n i f i c a n t thermald e g r a d a t i o n was under way f o l l o w i n g removal of t h e black s p r u c e cover some 2 y e a r s e a r l i e r . Near t h e e a s t e r n s i d e of t h e c l e a r e d right-of-way, advanced d e g r a d a t i o n was e v i d e n t a l o n g t h e r o u t e of t h e former Canadian N a t i o n a l Telegraph l i n e . Here t h e ground was thawed t o d e p t h s of 2 m o r more.
The a n n u a l p r o g r e s s of t h e z e r o i s o t h e r m w i t h i n t h e embankment and i n t h e u n d e r l y i n g ground i s shown f o r t h e c e n t e r l i n e p r o f i l e i n F i g u r e 8. Although t h e term "zero isotherm" i s used h e r e and elsewhere f o r convenience, t h e graph, i n f a c t , shows t h e i s o t h e r m f o r -0.15OC, t h i s v a l u e being c o n s i s t e n t w i t h t h e z e r o - c u r t a i n " c a l i b r a t i o n " of F i g u r e 5. The i s o t h e r m h i s t o r i e s were computed from t h e a d j u s t e d d a i l y mean t e m p e r a t u r e s w i t h no temporal o r s p a t i a l smoothing. T h i s e x p l a i n s some of t h e a p p a r e n t s c a t t e r i n t h e curves. Note a l s o t h a t t h e c u s p o i d a l shapes, s e e n e s p e c i a l l y i n t h e advancing thaw f r o n t s , a r e merely a r t e f a c t s of t h e method ( l i n e a r i n t e r p o l a t i o n ) used t o c a l c u l a t e t h e p o s i t i o n of t h e isotherm. T h i s procedure w i l l n e c e s s a r i l y o v e r e s t i m a t e i s o t h e r m d e p t h , e x c e p t when t h e i s o t h e r m t r a v e r s e s a measurement l e v e l .
It can be s e e n from F i g u r e 8 t h a t p r i o r t o embankment c o n s t r u c t i o n t h e subgrade thawed t o a depth of l e s s t h a n 1 m. S i n c e c o n s t r u c t i o n , t h e thaw h a s p r o g r e s s e d t o s u c c e s s i v e l y g r e a t e r d e p t h s each y e a r s o t h a t by 1980 t h e t o t a l thaw reached
1.4 m.
Annual mean t e m p e r a t u r e s computed from t h e 3 y e a r s of n e a r l y continuous r e c o r d s a r e shown f o r t h e embankment and subgrade c e n t e r l i n e p r o f i l e i n
- 3
76 I 1 78 7 9 80 81
TIME, y e a r s
FIGURE 8 C e n t e r l i n e z e r o isotherm.
F i g u r e 9 and s h o u l d e r p r o f i l e i n F i g u r e 10. I n 1979 and 1980, a n n u a l mean t e m p e r a t u r e s a t t h e embankment s u r f a c e were approximately +1.5OC on c e n t e r l i n e and +3'C a t t h e s h o u l d e r . A s u b s t a n t i a l i n c r e a s e ( a b o u t 1°C) h a s o c c u r r e d i n t h e upper 1-2 m of t h e subgrade s i n c e t h e embankment was b u i l t . A s i m i l a r t r e n d i s s e e n i n t h e embankment s h o u l d e r p r o f i l e ( F i g u r e 10). While s h a l l o w subgrade t e m p e r a t u r e s under t h e
s h o u l d e r a r e n o t more t h a n 0.3OC warmer t h a n t h o s e a t t h e c e n t e r l i n e , n e a r t h e s u r f a c e t h e d i f f e r e n c e i s s u b s t a n t i a l l y g r e a t e r . The r e v e r s e d c u r v e i n t h e upper l e v e l s of t h e 1977 p r o f i l e s s e e n i n b o t h F i g u r e s 9 and 10 r e f l e c t s t h e f a c t t h a t c o n s t r u c t i o n a c t i v i t i e s took p l a c e d u r i n g t h e f i r s t h a l f of t h a t y e a r . TEMPERATURE, 'C
FIGURE 9 Annual means: c e n t e r l i n e .
F i g u r e 11 shows t h e a n n u a l mean temperatures f o r t h e deep datum c a b l e l o c a t e d i n t h e nearby p a r t i a l l y c l e a r e d bush. It s h o u l d be n o t e d t h a t t h e
TEMPERATURE. OC
FIGURE 10 Annual means: shoulder.
TEMPERATURE. O C
FIGURE 11 Annual means: deep datum.
r e s u l t s f o r 1979 a r e n e a r l y i d e n t i c a l t o t h o s e f o r
1977. This may be i n t e r p r e t e d a s evidence t h a t t h e l a r g e d i f f e r e n c e s between t h e temperatures f o r t h e s e y e a r s i n t h e embankment p r o f i l e s a r e indeed caused by t h e presence of t h e embankment and a r e not merely a r e f l e c t i o n of d i f f e r e n t weather conditions. Note, too, i n F i g u r e 11 t h a t i n s u c c e s s i v e y e a r s t h e near s u r f a c e temperatures a r e a f r a c t i o n of a degree warmer. I n a d d i t i o n , t h e r e i s some tendency f o r t h e a c t i v e l a y e r t o deepen, a s s e e n i n F i g u r e 12.
At a l l t h r e e s i t e s , annual mean temperatures e x h i b i t a s t r o n g tendency t o d e c r e a s e s h a r p l y through t h e a c t i v e l a y e r . T h i s may be, a t l e a s t i n p a r t , a m a n i f e s t a t i o n of t h e thermal r e c t i f i c a t i o n e f f e c t r e s u l t i n g from temperature-dependent thermal
I
- 3 I I I
I
76 7 1 7 8 7 9 80 8 1
T I M E , y e a r s
FIGURE 12 Deep datum z e r o isotherm.
c o n d u c t i v i t y . When t h e f r o z e n c o n d u c t i v i t y i s
g r e a t e r t h a n t h e thawed c o n d u c t i v i t y , a s i t i s h e r e , annual mean temperatures w i l l decrease w i t h depth. The f a c t t h a t p r o f i l e d i s t o r t i o n i s confined p r i m a r i l y t o t h e a c t i v e l a y e r s u g g e s t s t h a t thermal r e c t i f i c a t i o n i s indeed t h e p r i n c i p a l mechanism a t t h e s e s i t e s . P r o f i l e c u r v a t u r e may, i n a d d i t i o n , i n d i c a t e t h a t deep temperatures a r e n o t y e t i n e q u i l i b r i u m w i t h l o n g t e r m c o n d i t i o n s a t t h e s u r f a c e . The 1980 p r o f i l e a t t h e deep datum s i t e , Figure 11, s u g g e s t s t h a t mean temperatures w i l l probably c o n t i n u e t o r i s e somewhat and t h a t , not unexpectedly, t h i s datum s i t e i s f a r from being "undisturbed."
Figure 13 shows a comparison of temperatures a t 1 m d e p t h below t h e o r i g i n a l ground s u r f a c e f o r t h e embankment c e n t e r l i n e , t h e shoulder, and t h e deep datum s i t e . From t h e shape of t h e c u r v e s i t can be i n f e r r e d t h a t thawing d i d not occur a t t h e deep datum s i t e d u r i n g t h e f i r s t 2 y e a r s , b u t d i d occur i n 1979 and was more i n t e n s e i n 1.980. The d a t a a l s o i n d i c a t e t h a t some s l i g h t thawing occurred a t t h e shoulder p r o f i l e even before c o n s t r u c t i o n . There was no thawing p r i o r t o c o n s t r u c t i o n a t t h e c e n t e r l i n e 1 m l e v e l . F i n a l l y , i t should be noted t h a t t h e subgrade temperatures a r e warmer t h a n t h e datum v a l u e s n o t only i n w i n t e r but a l s o i n summer;
t h e shallow embankment enhances summer h e a t i n g w h i l e reducing w i n t e r cooling.
Table 1 p r e s e n t s approximate v a l u e s of "n-
f a c t o r s " f o r t h e s i t e . The n-factors a r e computed a s t h e r a t i o of t h e thawing o r f r e e z i n g i n d e x a t t h e road s u r f a c e ( t h a t is, beneath t h e snow cover) t o t h a t of t h e a i r . Surface temperatures were e s t i m a t e d by e x t r a p o l a t i o n from measurements s t a r t i n g a t a l e v e l 20 cm below t h e s u r f a c e . S i n c e t h e a i r temperature records a t t h e s i t e were incomplete, d a t a were e x t r a c t e d from t h e published r e c o r d s f o r F o r t Simpson (200 km s o u t h of t h e s i t e ) , t h e n e a r e s t r e g u l a r l y r e p o r t i n g s t a n d a r d
m e t e o r o l o g i c a l s t a t i o n . The v a l u e s i n T a b l e 1 should t h e r e f o r e be considered w i t h caution. They can, however, be used f o r r e l a t i v e comparisons. Table 1 implies t h a t t h e g r a v e l s u r f a c e had a
thawing n-factor only 16%-19% g r e a t e r t h a n t h a t of t h e o r i g i n a l t e r r a i n . The d i f f e r e n c e i s s m a l l e r t h a n expected, and t h i s may, i n p a r t , r e f l e c t d i s t u r b a n c e a t t h e datum s i t e .
Prior to 1978 this study was financed by the Mackenz ie Highway Envirodntal Working Group. After suspension of construction activities, financing was continued by the National Research Council of Canada. The author gratefully acknawledges the contribution of G.
H.
Johnston, whose help with site selection, and initial planningand field work was indispensable. A special thank- you is also due to T.
L.
White, J. C. Plunkett, W. G. Cooke, andT.
Ireson for their unstinting assistance in the field and with essential chores in the laboratory. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with theapproval of the director.
TIME, y e a r s
REFERENCE FIGURE 13 Comparison of temperatures at 1 m below
original surface.
TABLE 1 Estimated "n-factors".
Datum Thawing Index Freezing Index C/L
7- ('C-day) ('Caday) nt "f O t "f
CONCLUSION
The data suggest that for the climatic and soil conditions in the Fort Wrigley area, road
construction results in degradation of the
permafrost in spite of special precautions taken to minimize disturbance of the organic cover. For the present conditions of road operation (winter road) with infrequent traffic and limited snow removal, annual mean road surface temperatures are up to 3OC above the freezing point. Since there may, however, be a significant thermal rectification effect associated with temperature-dependent thermal properties at shallow levels, it is possible that mean temperatures at depth will remain negative.
The maximum thaw penetration is about 50% greater beneath the embankment than at the datum site. During the study period there was an evident tendency for the active layer to deepen with time. This tendency, however, in addition to implying a long-term increase in deep-ground temperatures, also mirrors the increase observed in summer air
temperatures; consequently, it cannot be stated unequivocally that the active layer beneath the embankment will continue to deepen significantly. Further monitoring of the site would be required to establish the long-term trends with certainty.
Not all the data have been examined at this time. Further analysis, including numerical model simulation, may shed light on the ambiguities noted in this paper and provide useful guidance in
designing future embankments in the Mackenzie Valley
Goodrich,
L.
E., 1979, Transient probe apparatus for soil thermal conductivity measurements,&
Proceedings of a Symposium on Permafrost Field Methods, 3 Oct. 1977, and PermafrostGeophysics, 4 Oct. 1977, NRCC Technical Memorandum No. 124, Ottawa, June 1979.
BIBLIOGRAPHY
Brown, J., and Berg, R.L. (Editors), 1980, Environmental engineering and ecological baseline investigations along the Yukon River- Prudhoe Bay Haul Road, CRREL Report No. 80-19, Sept. 1980, 187 pp.
Judge, A.S., 1973, The thermal regime of the Mackenzie Valley: observations of the natural state, Environmental Social Committee, Northern Pipelines Task Force on Northern Oil
Development, Report No. 73-38, Dec. 1973, 177 pp.
T h i s p a p e r , w h i l e b e i n g d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g R e s e a r c h , remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be r e p r o d u c e d i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e f r o m t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , O t t a w a , O n t a r i o , K1A 0R6.